// DerivePartKey derives an unique 256 bit key from an ObjectKey and the part index.
func (key ObjectKey) DerivePartKey(id uint32) (partKey [32]byte) {
	var bin [4]byte
	binary.LittleEndian.PutUint32(bin[:], id)

	mac := hmac.New(sha256.New, key[:])
	mac.Write(bin[:])
	mac.Sum(partKey[:0])
	return partKey
}

// SealETag seals the etag using the object key.
// It does not encrypt empty ETags because such ETags indicate